1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method of compensating for boil-off corruption in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy concerns have stimulated the development of alternative transportation fuels for use in automobiles. A flexible fueled vehicle is capable of operating on gasoline, alcohol, or any mixture of the two. Alcohol fuels include methanol and ethanol. When operating on different fuels, modifications to the engine are necessary because of the different characteristics of each fuel. For example, an engine operating on 85% ethanol (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel as compared to 100% gasoline. This is due to the lower energy content of the ethanol.
The ideal air to fuel ratio in an internal combustion engine is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Since alcohol fuels require a lower air/fuel ratio than pure gasoline at stoichiometric, the engine must be compensated for in the rich direction. The amount of compensation increases as the percentage of alcohol in the fuel increases.
Maintaining stoichiometric operation in a flexible fueled vehicle is made even more difficult since such vehicles experience a condition referred to in the art as "boil-off". That is, when cold starting a vehicle that is fueled with alcohol in cold ambient conditions, some of the injected fuel remains in a liquid state. This liquid fuel often accumulates in the engine oil. When the oil temperature reaches the boiling point of the alcohol, the alcohol within the oil begins to vaporize. After vaporization, the alcohol is introduced into the engine through the crank case ventilation system. These additional vapors in the engine can cause a high level of rich fuel corruption until all of the alcohol in the oil is burned off.
U.S. Pat. No. 5,520,162, entitled "Boil-off for a Flexible Fuel Compensation System", hereby expressly incorporated by a reference, provides a flexible fuel compensation system including a method of methanol-type boil-off compensation. The methanol boil-off compensation method includes an initialization routine, a run mode routine and a shut-down routine. According to the method, a plurality of flags are monitored to determine when the methanol boil-off compensation should be used. Although this and other prior art methods are effective for their particular applications, there is room for improvement in the art.
For instance, previously, there was no provision for allowing the percent alcohol content learning system of the motor vehicle to function without being corrupted by the temporary introduction of alcohol vapors into the engine. This corruption causes the fuel composition multiplier of the percent alcohol content system to be frozen at a potentially incorrect value. This may result in drivability deficiencies after the boil-off condition ceases. Also, if the fuel composition multiplier is determined before the boil-off condition occurs, no technique is provided to compensate for the additional alcohol introduced into the engine during the boil-off period. This may also result in drivability deficiencies. Finally, if the vehicle is shut down during a boil-off condition, no compensation is made for the boil-off condition of the engine when it is restarted.
In view of the foregoing, it would be desirable to provide a temporary fuel composition multiplier for low and high air-flows in the engine. Such a temporary fuel composition multiplier would yield additional fuel control during a boil-off condition. Also, the pre-boil-off fuel composition multiplier value could be saved for post-boil-off use. Further, the temporary fuel composition multiplier could be saved when the vehicle is shut down during a boil-off condition and re-used after the vehicle is restarted.